1. Field of the Invention
The present invention relates to a method of determining conditions for the growth of a plasma silicon nitride (SIN) film for use as an overcoat layer in a semiconductor device, especially a semiconductor memory such as an ultraviolet erasable programmable read-only memory (UV erasable PROM) or the like, and a method of manufacturing a semiconductor device having an overcoat layer in the form of a plasma SiN film, using such a method of determining conditions.
2. Description of the Prior Art
One ultraviolet erasable programmable read-only memory (UV erasable PROM) is shown in FIG. 1 of the accompanying drawings. As shown in FIG. 1, the ultraviolet erasable programmable read-only memory has a semiconductor substrate 1 of a first conductivity type, e.g., P type, with spaced source and drain regions 2, 3 of a second conductivity type, e.g., N type, being formed in one surface of the semiconductor substrate 1. A floating gate 5 is disposed partly over and extends between the source and drain regions 2, 3 with an insulation film 4A interposed therebetween. A control gate 6 is disposed over the floating gate 5 with an insulation film 4B interposed therebetween. Electrodes 7, 8 are deposited respectively on the source and drain regions 2, 3. An overcoat layer 9 is deposited on the surface formed so far. Usually, the overcoat layer 9 comprises a silicon nitride (SIN) film formed by plasma chemical vapor deposition (CVD).
As shown in FIG. 2 of the accompanying drawings, a plasma CVD apparatus comprises a chamber 11 housing an upper electrode 13 which has gas nozzles 12 for supplying raw material gases of SiH.sub.4, NH.sub.3, and a heater 14 positioned below the upper electrode 13 in confronting relationship thereto. A semiconductor wafer 16, on which a desired plasma silicon nitride film is to be deposited, is placed on the heater 14 through a susceptor 15 doubling as a lower electrode. In operation, a high-frequency electric energy is applied between the upper electrode 13 and the susceptor 15 by a high-frequency power supply 18, and the raw material gases are introduced into the chamber 11 through the gas nozzles 12. As a result, an SiN film is formed on the semiconductor wafer 16 by plasma chemical reaction, i.e., plasma CVD. The chamber 11 has discharge ports 17 opening downwardly.
The plasma SiN film thus formed, which is used as the overcoat layer 9 of the UV erasable PROM, is required to have a high ultraviolet radiation transmittance for erasing any stored data, unlike other semiconductor devices. To meet such a requirement, it is necessary to reduce the number of Si--H bonds in the SiN film.
Normally, the number of Si--H bonds can be reduced if the flow rate of SiH.sub.4 is reduced. However, if the flow rate of SiH.sub.4 is reduced, then the internal stress of the SiN film is increased. Consequently, it is difficult to determine conditions with a margin for increasing the ultraviolet radiation transmittance and also maintaining a film stress in a range low enough to be free of reliability problems, simply by controlling the parameter of the SiH.sub.4 flow rate.